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Down quark |
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| Composition: | Elementary particle |
| Family: | Fermion |
| Group: | Quark |
| Generation: | First |
| Interaction: | Strong, Weak, Electromagnetic force, Gravity |
| Antiparticle: | Down antiquark (d) |
| Theorized: | Murray Gell-Mann (1964) George Zweig (1964) |
| Discovered: | SLAC (1967) |
| Symbol(s): | d |
| Mass: | 3.5–6.0 MeV/c2 |
| Decays into: | Up quark |
| Electric charge: | −1⁄3 e |
| Color charge: | Yes |
| Spin: | 1⁄2 |
The down quark is a first-generation quark with a charge of -(1/3)e. It is the second-lightest of all the six flavoured quarks. Its bare mass is not well determined, but probably lies between 4 and 8 MeV. According to the Standard Model of particle physics, it and the up quark are the fundamental constituents of the nucleons; the proton contains one down quark and two up quarks, while the neutron contains two down quarks and one up quark. Note, however, that the majority of the mass in nucleons comes from the energy in the gluon field holding the quarks together, and not the quark masses themselves.
Down quarks were named when Gell-Mann and Zweig developed the quark model in 1964, and the first evidence for them was found in deep inelastic scattering experiments at SLAC in 1967.
Some of the hadrons containing down quarks include:
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